The present invention relates to a process for producing a free hydroxylamine aqueous solution, in more detail, a process for producing a free hydroxylamine aqueous solution, in which free hydroxylamine is obtained by the reaction between hydroxylammonium sulfate and ammonia, comprising the steps of mixing a lower alcohol to precipitate and separate the sulfates containing ammonium sulfate produced by the reaction; mixing an alkali with the separated ammonium sulfate, to produce ammonia, and recovering and recycling the produced ammonia.
Hydroxylamine is industrially widely used as a medical and agricultural intermediate raw material or as a metal surface treating agent, etc.
However, since free hydroxylamine is very unstable and is easily decomposed, it is generally converted into a relatively stable hydroxylamine salt (hydroxylammonium salt) for use. The decomposition of free hydroxylamine is especially likely to occur, for example, in the presence of heavy metal ions, at a high concentration, in a strong alkali or at a relatively high temperature.
When hydroxylamine is actually used in the above applications, free hydroxylamine, not a hydroxylammonium salt, is preferably used, and an aqueous solution containing free hydroxylamine at a higher concentration is often required. Such a free hydroxylamine aqueous solution has been obtained by several methods.
As a recent method for obtaining free hydroxylamine, the reaction between a hydroxylammonium salt and an alkali is used. For example, in the case of the reaction between hydroxylammonium sulfate and ammonia, after completion of reaction, the reaction system contains a large amount of ammonium sulfate in addition to the intended free hydroxylamine. So, it is proposed to separate free hydroxylamine from ammonium sulfate, for obtaining a highly pure and highly concentrated free hydroxylamine solution.
In the method described in U.S. Pat. No. 4,956,168, the ammonium sulfate precipitated in the reaction between hydroxylammonium sulfate and ammonia in an alcohol is removed, to obtain a free hydroxylamine alcohol solution. The alcohol solution is then mixed with an acid other than sulfuric acid, to synthesize a hydroxylammonium salt other than hydroxylammonium sulfate.
Furthermore, German Patent Publication No. 1247282 and German Patent Publication No. 3601803 (Japanese Patent Laid-Open (Kokai) No. Sho62-171905, U.S. Pat. No. 4,778,669) describe methods for obtaining a free hydroxylamine alcohol solution by letting hydroxylammonium and ammonia react with each other in an alcohol and removing the precipitated ammonium sulfate.
Furthermore, in German Patent Publication No. 19547758 (U.S. Pat. No. 5,837,107), hydroxylammonium sulfate and an alkali are caused to react with each other in water not containing any alcohol, and the precipitated salt is removed. Then, the reaction solution is supplied into an intermediate stage of a column, and separated into a hydroxylamine aqueous solution not containing any salt discharged from an upper stage and an aqueous solution containing a salt discharged from a lower stage.
In the conventional methods for obtaining free hydroxylamine by the reaction between hydroxylammonium sulfate and ammonia, it is mainly intended to separate the free hydroxylamine and ammonium sulfate produced by the reaction, but what will happen with the separated ammonium sulfate is not discussed.
Especially if the hydroxylammonium sulfate used for the reaction is obtained according to the Raschig""s method, recently a purchased sodium nitrite aqueous solution is often used as a raw material, instead of ammonium nitrite. In this case, the sulfates obtained after completion of reaction contain not only ammonium sulfate but also the sodium sulfate contained in the raw material. When the sulfates are a mixture of them, it is very difficult to separate ammonium sulfate from sodium sulfate, and it is not appropriate to use the mixture as a valuable substance. If the sulfates are dissolved to be dumped as waste water, an environmental problem arises since ammonium sulfate contains nitrogen atoms.
As described above, if hydroxylammonium sulfate is treated by ammonia, a large amount of sulfates containing ammonium sulfate are produced. Hitherto, the sulfates are separated and removed beforehand, to simplify the subsequent operation of obtaining free hydroxylamine. When the ammonium sulfate in the separated sulfates is dumped as waste water, any treatment for environmental conservation is necessary since the waste water contains nitrogen atoms.
An object of the present invention is to provide a process for producing a free hydroxylamine aqueous solution, in which the sulfates separated like this are caused to react with an alkali, to be converted into ammonia and sulfates not containing any nitrogen atom, for recovering valuable ammonia from them, to recycle it in the reaction system for effective use.
Another object of the present invention is to provide a process for producing a free hydroxylamine aqueous solution, which can be effectively used when the sulfates obtained after completion of reaction are a mixture consisting of ammonium sulfate and sodium sulfate difficult to separate.
Other objects of this invention will be clarified in the following description.
This invention which achieves the above objects is a process for producing a free hydroxylamine aqueous solution, in which a free hydroxylamine aqueous solution is obtained by letting an aqueous solution or slurry containing hydroxylammonium sulfate and a gas mainly containing ammonia react with each other, mixing a solution mainly containing a lower alcohol with the produced aqueous solution or slurry, and separating the precipitated sulfates containing ammonium sulfate, comprising the steps of mixing an alkali with the separated sulfates, to produce ammonia gas containing water, and recycling the ammonia in the reaction system.
This invention also includes the following effective modes.
(a) As the aqueous solution or slurry containing hydroxylammonium sulfate, some or all of the hydroxylammonium sulfate containing sodium sulfate obtained according to the Raschig""s method is used.
(b) The alkali is sodium hydroxide or calcium oxide and/or calcium hydroxide.
(c) As sodium hydroxide, a sodium hydroxide aqueous solution is used.
(d) As the calcium oxide and/or calcium hydroxide, a calcium hydroxide slurry is used.
(e) The sulfates containing ammonium sulfate are formed into an aqueous solution, and an alkali is mixed with it.
(f) A solution mainly containing a lower alcohol is mixed with the reaction solution remaining after separating and removing the sulfates containing ammonium sulfate precipitated in the ammonia reaction.
(g) The sulfates obtained by separation and an alkali are mixed, to produce ammonia which is recovered and recycled in the reaction system.
(h) Powdery hydroxylammonium sulfate is added to the reaction solution remaining after separating and removing the sulfates containing ammonium sulfate precipitated in the ammonia reaction, and ammonia is again caused to react.
(i) The operation of the above (h) is repeated plural times.
(j) The reaction solution obtained in any of the above (f) to (i) and a solution mainly containing a lower alcohol are mixed.
(k) The lower alcohol is methanol.
(l) The reaction between hydroxylamine sulfate and ammonia is effected in the presence of a hydroxylamine stabilizer.
(m) The mixing of the sulfates and the alkali is effected at room temperature or higher temperature.
(n) Nitrogen gas or air is blown into the mixed solution consisting of the sulfates and the alkali, and the ammonia produced by the reaction is obtained by degasification.